Heat exchanger

ABSTRACT

A heat exchanger of the type adapted for use in generating steam in a nuclear power plant, and a method for manufacturing the same. A plurality of curved tubular walls are equidistantly arranged with respect to each other around a central axis with each of these walls having an inner concave side directed toward the axis and an outer convex side directed away from the axis and with each of the walls including an elongated tube having a hollow interior through which a heat-exchanging medium is adapted to flow. Each of the walls further includes a fin structure fixed to the tube thereof at least partially along the tube for forming a vibration-resistant, unitary tubular wall structure. The heat exchanger is manufactured by curving a plurality of finned tubes and welding the tubes to each other to form a tubular wall.

United States Patent 1 Beckmann et al.

HEAT EXCHANGER Inventors: Georg Beckmann, Vienna; Kurt Fritz, Klosterneuburg; Josef Lippitsch, Graz, all of Austria Waagner-Biro Aktiengesellschaft, Vienna, Austria 1 Filed: Aug. 2, 1972 Appl. No.: 277,285

Assignee:

Foreign Application Priority Data Aug. 3, 1971 Austria 674 8/71 US. Cl 165/162, 122/510 Int. Cl F281 9/00 Field of Search 165/76, 162, 158; 122/32,

References Cited UNITED STATES PATENTS FOREIGN PATENTS OR APPLICATIONS 581,742 10/1946 Great Britain 165/164 Primary Examiner-Manuel A. Antonakas Assistant ExaminerTheophil W. Streule, Jr. Attorney, Agent, or FirmSteinberg & Blake 57 ABSTRACT A heat exchanger of the type adapted for use in generating steam in a nuclear powerplant, and a method for manufacturing the same. A plurality of curved tubular walls are equidistantly arranged with respect to each other around a central axis with each of these walls having an inner concave side directed toward the axis and an outer convex side directed away from the axis and with each of the walls including an elongated tube having a hollow interior through which a heatexchanging medium is adapted to flow. Each of the walls further includes a fin structure fixed to the tube thereof at least partially along the tube for forming a vibration-resistant, unitary tubular wall structure. The heat exchanger is manufactured by curving a plurality of finned tubes and welding the tubes to each other to form a tubular wall.

12 Claims, 8 Drawing Figures HEAT EXCHANGER BACKGROUND OF THE INVENTION The present invention relates to heat exchangers.

In particular, the present invention relates to heat exchangers which are adapted to be used as steam generators for nuclear power plants.

The present invention relates particularly to that type of heat exchanger where a heat-exchanging medium flows through a plurality of tubes along similar flow paths.

It is already known to provide heat exchangers in which the tubes are suspended by way of suitable lugs, rods, bars, and the like. Such a suspension, however, requires a large amount of space and for this reason tube-supporting structures of this type are not suitable for heat exchangers in which the tubes are located very close to each other. Furthermore, with conventional structures of the above type it is not possible to calculate, or it is only possible to approximate, the forces with which the tubes are gripped particularly in their operating condition when the tubes are hotter than in their cold condition. As a result of this latter factor, a

calculation of the natural frequency of the tubes is not possible. It then becomes extremely difficult to determine the natural frequency of the tubes by a corresponding arrangement of the tube-supporting structure in such a way that there will be no resonance as a result of other sources of vibration as, for example, as a result of the Karman eddy current separation or the acoustic frequency of the flow path. These difficulties often result in destruction of the heat exchanger or steam generator.

Moreover, a fixed gripping of the tubes is not permanently assured by way of tubular supports which clamp the tubes. Such clamping supports present a particular problem because of the friction wearing of the tubes at the clamping location and the cold welding thereof particularly in an inert gas or reducing atmosphere.

On the other hand, with a welded construction there are well-defined gripping relationships which lend themselves to use in the design of constructions requiring a small amount of space. The disadvantage of a tubular supporting structure, as is now used for boilers, where the individual tubes of a bundle of tubes are directly fixed by way of hand welding resides in the fact that this type of welding cannot be tested or can only be tested in an inadequate manner with respect to determination of the presence of faults which might lead to failure. This circumstance is not acceptable in the case of steam generators and heat exchangers which are to be used in nuclear power plants.

SUMMARY OF THE INVENTION It is accordingly a primary object of the present invention to provide a heat exchanger which will avoid the above drawbacks.

In particular it is an object of the present invention to provide a heat exchanger, and method for manufacturing the same, which will result in a construction which is practically without any frictional wearing and which is subjected only to an extremely small degree of vibrations.

It is especially an object of the invention to provide a heat exchanger which will enable these results to be achieved in a construction where the tubes of the heat exchanger are located extremely close to each other so that a large number of flow paths for the medium within the tubes can be situated in an extremely small volume of space.

Yet another object of the present invention is to provide a heat exchanger which enable the spacing between the tubes to be maintained in a predetermined manner which will assure proper flow for the heat exchanging medium which is at the exterior of the tubes, even though the latter are located extremely close to each other.

In accordance with the invention a heat exchanger, which is particularly adapted to be used as a steam generator for a nuclear power plant, is provided with a plurality of curved tubular walls which are equidistantly arranged with respect to each other around a central axis with each of these walls having an inner concave side directed toward this axis and an outer convex side directed away from this axis. Each of the walls include an elongated tube having a hollow interior through which a heat-exchanging medium is adapted to flow, and each of these walls further includes a fin means fixed to the tube thereof at least partially along the tube for forming a vibration-resistant, unitary wall structure.

According to the method of the invention, the tubes which can have fins adjoined thereto or which may be initially manufactured with fins are curved to a predetermined curvature and then are welded together so as to fonn according to this method of the invention the tubular walls of the invention.

BRIEF DESCRIPTION OF DRAWINGS The invention is illustrated by way of example in the accompanying drawings which form part of this application and in which:

FIG. 1 is a schematic plan view of one embodiment of a heat exchanger according to the invention;

FIG. 2 is a schematic plan view of another embodiment of a heat exchanger according to the invention;

FIG. 3 is a schematic fragmentary partly sectional elevation taken along line II of FIG. 1 or II-II of FIG. 2 and illustrating one possible embodiment of a structure for supporting the tubular walls;

FIG. 4 is a fragmentary partly sectional schematic elevation taken along line II of FIG. 1 and IIII of FIG. 2 and illustrating another possible embodiment of a structure for supporting the tubular walls of the invention; and

FIGS. 5-8 respectively illustrate in schematic fragmentary sectional views different embodiments of spacer means according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring first to FIG. 1, there is illustrated therein, in a schematic plan view, a heat exchanger in which a plurality of cylindrical tubular walls are concentrically arranged with respect to each other around a common axis which is normal to the plane of FIG. 1, with the several cylindrical tubular walls 4 being arranged as illustrated in dot-dash lines in FIG. 1. These tubular cylindrical concentric walls 4 are surrounded by an outer tubular cylindrical wall 3, while they extend around an inner cylindrical tubular wall 5. The heat-exchanging medium which flows around the tubular walls at the exterior thereof flows in a direction perpendicular to the plane of FIG. 1 so as to flow in a direction which is parallel to the central axis of the heat exchanger. The plurality of concentric cylindrical tubular walls 3-5 are situated within a pressure vessel 12. A supporting frame means is formed by a plurality of bars 8 which are fixed, as by welding, to the inner surface of the pressure vessel 12 and which extend radially with respect to the central axis, being uniformly distributed about the central axis in a manner shown in FIG. 1. The several bars 8 of the supporting frame means are fixed, as by welding, at their inner ends to a cylindrical fluiddisplacement pipe 13 situated at the center of the pressure vessel 12 and thus being carried by the bars 8. Thus, the inner fluid-displacing body 13 and the outer pressure vessel 12, which are coaxial and have a common axis coinciding with the central axis of the heat exchanger define between themselves the space within which the tubular walls 3-5 are concentrically arranged.

In the embodiment of the invention which is schematically illustrated in a plan view in FIG. 2, the heat exchanger has tubular walls 1 and 2 which in a plane perpendicular to the central axis of the heat exchanger have the configuration of an involute curve. With this particular construction the tubular walls 1 and 2 which have the involute curvature have tubes all situated in planes normal to the central axis of the heat exchanger while the connecting tubes 14, which are approximately of a semicircular configuration, extend from the end of one tube of involute wall 1 or 2 down to the opposed involute wall tube which is located in the next lower horizontal plane. In contrast with this construction, however, it is also possible to provide a construction where the tubes of FIG. 2 have a constant pitch so that they progress in a helical manner vertically in an axial direction.

It is to be noted that in both embodiments of the invention, illustrated respectively in FIGS. 1 and 2, the tubular walls are curved so as to have inner concave sides directed toward the central axis of the heat exchanger and outer convex sides directed away from the central axis of the heat exchanger. Moreover, the several tubular walls are all equidistantly arranged with respect to each other, in a radial direction with respect to the central axis. Thus, in the case of FIG. 1 the several concentric cylindrical tubular walls 3-5 are all situated equidistantly from each other, and the same is true of the involute tubular walls 1 and 2 of FIG. 2.

FIGS. 3 and 4 schematically illustrate in fragmentary sectional elevations different embodiments, respectively, of arrangements according to which the supporting frame means 8 supports the several tubular walls. It is to be understood that both the construction of FIG. 3 and that of FIG. 4 is applicable both to FIG. 1 and to FIG. 2.

In the embodiment of FIG. 3 the several tubular walls are suspended in such a way that they hang from the supporting frame means. Thus, with this embodiment the several tubular walls 1-5 are welded at their top ends to the several bars 8 of the supporting frame means so as to hang downwardly from the latter. Thus, by being welded in this way to the supporting frame means the distance between the several tubular walls is determined, and in addition a spacer means 11 is provided for further determining the distance between the several tubular walls. The use of the several spacer elements 11 in the manner illustrated in FIG. 3 further serves to damp vibrations in the tubular walls. As is particularly apparent from the left end of the bar 8 shown in FIG. 3, the several bars 8 are fixed in cantilever fashion at their outer ends-to the inner surface of the outer vessel 12 by way of suitable supporting brackets 15 which are fixed on the one hand to the vessel 12 at the inner surface thereof and which on the other hand are fixed with the several bars 8, respectively, at their outer ends. As was indicated above, the several bars 8 of the supporting frame means in turn carry at their inner ends the displacement pipe 13 which is coaxial with the several tubular walls l-S. Through the pipe 13 are guided the several connecting tubes which respectively communicate with the different heating units of the heat exchanger. Furthermore, in order to avoid a short circuit in the flow paths, the pipe 13 is tightly closed at a given location so that the fluid medium in which the pipe 13 is located stagnates in the pipe 13.

With the arrangement shown in FIG. 4, the several tubular walls 1-5 rest on the supporting frame means formed by the bars 8, with these tubular walls being fixed at their bottom ends, as by welding, to the several bars 8 so that in this case the several tubular walls extend upwardly from the supporting frame means. The several bars 8 in this embodiment are carried by the pressure vessel 12 and are connected with the pipe 13 in the manner described above in connection with FIG. 3. Thus, the supporting frame means takes the form of a carrier grid having its several bars extending radially from the pipe 13 out to the vessel 12.

In the embodiment of FIG. 4, the several tubular walls l-S are spaced from each other by way of a spacer means composed of spacer components 16 which have a mushroom-shaped configuration. These spacer elements 16 are arranged in such a way that there are fixed at their smaller ends to the fin means 7 which is in turn fixed to the several tubes of the several walls, extending between these tubes, and the enlarged head ends of the several mushroom-shaped spacer elements 16 are directed toward each other in pairs, as illustrated in FIG. 4, so that these enlarged head ends of each pair of opposed spacer elements 16 will be able to engage each other, if necessary, while their smaller foot ends are respectively welded to the several fin means 7 which extend between the several tubes of the several tubular walls. Thus, with the construction of this type it is possible for each of the individual tubular walls to be introduced in an axial direction into the assembly.

FIG. 5 illustrates one embodiment of a spacer means used with the tubular walls of the heat exchanger of the invention. As is apparent from FIG. 5, each of the tubular walls is composed of an elongated tube 6 which has a fin means 7 fixed thereto as by being welded to the tube 6 along the length thereof. Each pair of fins located between a pair of successive tube convolutions 6 of each wall are also welded to each other so as to form in this way the vibration-resistant unitary tubular wall structure of each of the tubular walls of the heat exchanger of the invention. As is apparent from FIG. 5 every other tubular wall has the fin means 7 which extends between every other pair of convolutions formed with a bore passing therethrough. Thus it will be noted that in the sequence illustrated in FIG. 5 the left and right tubular walls have fins formed with openings passing therethrough in order to accommodate the elongated spacer components 9, while the intermediate tubular wall of FIG. 5 does not have any bores or openings passing through the fins thereof. Thus, with this arrangement the alternating tubular walls, or in other words every second tubular wall, will have at the fin means thereof openings for accommodating the spacer means 9. Also, in each of the walls which carries the spacer means the fin means situated between alternating pairs of successive convolutions of the tube 6 are formed with openings to accommodate the spacer means 9. In this embodiment the spacer means 9 takes the form of a plurality of radially extending elongated pins or rods 9 which respectively extend through the openings which are formed in the fin means 7 and which are welded to the fin means 7 in such a way that successive coaxial spacer pins 9 which extend along a common radius of the heat exchanger have ends directed toward each other and situated close to a fin means between a pair of tubular convolutions of a tubular wall means between the alternating pair which carry the spacer means 9, and in the same way this same pair of pins 9 project at their outer ends into the space between a pair of tubular convolutions of the next inner and next outer tubular walls which are not illustrated in FIG. 5. Thus, each of the elongated spacer components 9 of this embodiment has a length corresponding to twice the distance between successive tubular walls. Thus, the alternating tubular walls of the arrangement shown in FIG. 5 are to be considered as having a maximum thickness equal to double the distance between the successive tubular walls so that it is necessary to introduce into or remove from the assembly, as required, a tubular wall structure having this maximum thickness. As was indicated above, FIG. 5 shows the details of the structure of the individual tubular walls. Thus, each individual tubular wall is made up of a number of tubular portions 6. Initially the tubular portions 6 are straight and a pair of elongated fins 7 are welded axially toeach of the tubular portions 6 at diametrically opposed sides thereof. After the fins are welded in this manner to the tubes, and this welding can take place mechanically, the welding seam or weldment is tested so as to make certain that no reduction in the cross-sectional area of any tube has occurred. Then, the tested tubes are curved along portions of a helix and are then welded together in end-to-end relation so as to form a continuous spiral with the edges of the fins located next to each other in order to form a unitary wall structure. The adjoining edges of the adjoining fins are also welded to each other so as to form a strong unitary curved tubular wall structure. In the case of cylindrical tubular walls as shown in FIG. 1, each wall has a continuous tube extending spirally around the axis of the tubular wall. In the case of the involute tubular walls of FIG. 2, it is possible to simplify the construction by first forming a flat tubular wall with all of the fins joined to each other and then curving this flat wall into the shape of an involute. Of course, instead of utilizing tubes to which the fins are welded it is possible to utilize finned tubes which are integrally formed with fins when they are initially manufactured, and then such finned tubes are correspondingly curved and welded as described above. The advantage of joining the fins to the tubes by welding the fins thereto is that with such construction it is not essential to provide fins which extend completely along the entire length of the tubes. Instead the fins may be distributed along the tubes with spaces being situated between successive fins, so that in the completed tubular wall there are passages or openings extending through the tubular wall enabling the heat-exchanging medium at the exterior of the tubular walls to flow through these openings in order to mix portions of this outer heat-exchanging me dium located at different passages defined between successive tubular walls.

In the embodiment of the invention which is illustrated in FIG. 6, the spacer means has a construction according to which shorter pins or studs 9 are respectively welded at one of their ends to fins in the manner shown in FIG. 6, with these spacer components 9 also extending radially with respect to the central axis of the heat exchanger. Thus at predetermined locations along the fins of each tubular wall spacer pins 9 are welded in the manner shown in FIG. 6. The length of the several spacer pins corresponds to the clearance between I fins of successive tubular walls so that these spacer pins serve to determine the distance between the pairs of successive tubular walls. In this construction allot the spacer pins 9 are fixed either at their outer ends to the fins and project from their outer ends inwardly toward the central axis of the heat exchanger or they are fixed at their inner ends to the several fins and project outwardly away from the central axis of the heat exchanger.

In order to assemble the several tubular walls having a structure as shown in FIG. 6 or a structure as shown in FIG. 5, the successive tubular walls are threaded into the space surrounded by the previously assembled tubular wall. Assuming that the tubular walls of FIGS. 5 and 6 form the cylindrical walls as shown in FIG. 1, then it is clear that at each tubular wall the space between the spiral tubular convolutions thereof also forms a spiral similar to a screw thread extending around the axis of the tubular wall. Therefore, if it is assumed that the left tubular wall of FIG. 5, for example, has first been assembled into the heat exchanger, then the intermediate tubular wall of FIG. 5 can be threaded with respect to the left spacer components 9 until this intermediate wall is surrounded by the left wall of FIG. 5, and then the next tubular wall shown at the right of FIG. 5 can be threaded into the position shown in FIG. 5 with respect to the intermediate wall. In the same way, in FIG. 6, each tubular wall can be threaded into its concentric relationship with respect to the immediately preceding tubular wall which has just been introduced into the heat exchanger. In this way it becomes possible to fix the spacer pins 9 to the tubular walls prior to assembly thereof into the heat exchanger with simple threading of the successive tubular walls into the heat exchanger taking place in the above-described manner.

According to the embodiment of the invention which is illustrated in FIG. 7, the spacer means takes the form of a plurality of bolts 10 which serve to determine the distance between successive pairs of tubular walls.

With this construction after a pair of tubular walls are assembled, openings are drilled through the fins thereof at several locations and the openings of the inner cylindrical wall are threaded while the bolts 10 are passed through the openings of the outer tubular wall and threaded into the threaded openings of the inner tubular wall. Then the heads of the bolts 10 are welded to the fins of the outer tubular wall. In this way the successive fixing of the tubular walls to each other by the spacer means 10 which maintains the successive tubular walls equidistantly spaced from each other is carried out. As a result of the welding of the heads of the bolts to the fins it is not possible for the bolts or screws to become loose during operation of the heat exchanger.

The arrangement which is illustrated in FIG. 8 is similar to FIG. 7. The difference is that in this embodiment instead of using headed screws 10 threaded studs are used. These threaded studs 10 of FIG. 8 are assembled with the remaining structure in the manner described above in connection with FIG. 7. Thus, after the threaded end portion of a stud 10 is threaded into a threaded opening formed in a fin 7 of one tubular wall, the opposite end of the stud 10 which extends through the opening of the adjoining tubular wall is welded directly to the fin structure of the latter wall so that in this way the studs 10 of FIG. 8 serve the same purpose as the bolts 10 of FIG. 7.

Thus, with the above-described structure of the invention it will be seen that the several tubular walls are each composed of tubular portions extending either along elements of a cylinder or along elements of a wall having an involute curvature, and these several tubular portions are welded to each other to form the unitary tubular wall structure. It is furthermore to be noted that the several spacer pins of FIGS. 5 and 6 may be fixed to the tubular walls prior to curving thereof. In other words, as was pointed out above it is possible to form the tubular walls by first fixing the fins to the tubes in the event that the tubes are not initially manufactured with the fins as an integral part thereof. Then the several finned tubes are welded to each other at the free edges of their fins in order to form a flat tubular panel. While such a panel is still in its flat condition it is possible to fix the spacer pins 9 thereto in a manner described above and shown in FIGS. 5 and 6, and then after these spacer pins are fixed to the flat panels it is possible to curve the latter so as to form either part of a cylinder or so as to have the involute curvature of FIG. 2. It is also possible to fix the pins 9 to the fins of the tubes prior to curving of the individual finned tubes with the pins then being joined to the fin of an adjoining tubular portion when the several tubular portions are assembled. It will be noted that in allof the abovedescribed embodiments the tubular walls extend axially of the heat exchanger and have the distance therebetween determined by the spacers which extend transversely with respect to the tubular walls.

What is claimed is:

ll. In a heat exchanger particularly for use as a steam generator for a nuclear power plant, a plurality of curved tubular walls equidistantly arranged with respect to each other around a central axis with each of said walls having an inner concave side directed toward said axis and an outer convex side directed away from said axis, each of said walls including an elongated tube having a hollow interior through which a heatexchanging medium is adapted to flow and each of said walls further including a fin means fixed to said tube thereof at least partially along said tube for forming a vibration-resistant, unitary tubular wall structure, each of said tubular walls including a plurality of tubular portions which are axially spaced from each other and which are welded one to the next by way of said fin means.

2. The combination of claim 1 and wherein the individual tubes of said tubular walls extend transversely with respect to the direction of flow of a heatexchanging medium which flows around said tubular walls at the exterior thereof, each of said tubular walls having tubular portions between which said fin means is located, and spacer means operatively connected with said tubular walls for spacing the latter equidistantly from each other in a radial direction with respect to said central axis.

3. Thecombination of claim 1 and wherein each of said tubular walls extends along an involute curve in a plane normal to said central axis.

4. The combination of claim 1 and wherein said tubular walls respectively have the configuration of cylinders and are concentrically arranged with respect to said axis with the tube of each of said tubular walls extending spirally around said axis.

5. The combination of claim 1 and wherein a supporting frame means is welded to said fin means for supporting said tubular walls.

6. The combination of claim 5 and wherein said supporting frame means is located over said tubular walls so that the latter extend downwardly from said supporting frame means.

7. The combination of claims, and wherein said supporting frame means is situated beneath said tubular walls so that the latter rest on said frame means.

8. The combination of claim 2 and wherein said spacer means includes a plurality of screws radially arranged with respect to said central axis and fixed to adjoining tubular walls for spacing the latter from each other as well as fixing them to each other.

9. The combination of claim 8, and wherein each screwv is fixed to a pair of said walls with one of said pair of walls forming an inner wall while the other of said pair of walls forms an outer wall situated at a greater distance from said central axis than said inner wall, each screw being threaded to the fin means of said inner wall and being welded to the fin means of said outer wall.

10. The combination of claim 2 and wherein said spacer means includes a plurality of elongated radially extending spacer elements welded to said fin means of said walls and determining the radial distance therebetween.

ll. The combination of claim 1 and wherein each of said tubular walls has a pair of opposed ends and frame means welded to said walls at one of the ends thereof for fixing the walls to each other and determining the radial spacing therebetween, and spacer means carried by said walls for determining the distance therebetween at locations beyond said frame means.

12. The combination of claim I and wherein an inner pipe has an axis coinciding with said central axis and an outer cylinder coaxially surrounds said inner pipe and defines therewith a space in which said tubular walls are located, and a plurality of supporting bars extending radially between and being fixed to said inner pipe and outer cylinder, said bars forming a supporting frame means and said plurality of tubular walls being fixed at one of the ends thereof to said supporting frame means. 

1. In a heat exchanger particularly for use as a steam generator for a nuclear power plant, a plurality of curved tubular walls equidistantly arranged with respect to each other around a central axis with each of said walls having an inner concave side directed toward said axis and an outer convex side directed away from said axis, each of said walls including an elongated tube having a hollow interior through which a heat-exchanging medium is adapted to flow and each of said walls further including a fin means fixed to said tube thereof at least partially along said tube for forming a vibration-resistant, unitary tubular wall structure, each of said tubular walls including a plurality of tubular portions which are axially spaced from each other and which are welded one to the next by way of said fin means.
 2. The combination of claim 1 and wherein the individual tubes of said tubular walls extend transversely with respect to the direction of flow of a heat-exchanging medium which flows around said tubular walls at the exterior thereof, each of said tubular walls having tubular portions between which said fin means is located, and spacer means operatively connected with said tubular walls for spacing the latter equidistantly from each other in a radial direction with respect to said central axis.
 3. The combination of claim 1 and wherein each of said tubular walls extends along an involute curve in a plane normal to said central axis.
 4. The combination of claim 1 and wherein said tubular walls respectively have the configuration of cylinders and are concentrically arranged with respect to said axis with the tube of each of said tubular walls extending spirally around said axis.
 5. The combination of claim 1 and wherein a supporting frame means is welded to said fin means for supporting said tubular walls.
 6. The combination of claim 5 and wherein said supporting frame means is located over said tubular walls so that the latter extend downwardly from said supporting frame means.
 7. The combination of claim 5, and wherein said supporting frame means is situated beneath said tubular walls so that the latter rest on said frame means.
 8. The combination of claim 2 and wherein said spacer means includes a plurality of screws radially arranged with respect to said central axis and fixed to adjoining tubular walls for spacing the latter from each other as well as fixing them to each other.
 9. The combination of claim 8, and wherein each screw is fixed to a pair of said walls with one of said pair of walls forming an inner wall while the other of said pair of walls forms an outer wall situated at a greater distance from said central axis than said inner wall, each screw being threaded to the fin means of said inner wall and being welded to the fin means of said outer wall.
 10. The combination of claim 2 and wherein said spacer means includes a plurality of elongated radially extending spacer elements welded to said fin means of said walls and determining the radial distance therebetween.
 11. The combination of claim 1 and wherein each of said tubular walls has a pair of opposed ends and frame means welded to said walls at one of the ends thEreof for fixing the walls to each other and determining the radial spacing therebetween, and spacer means carried by said walls for determining the distance therebetween at locations beyond said frame means.
 12. The combination of claim 1 and wherein an inner pipe has an axis coinciding with said central axis and an outer cylinder coaxially surrounds said inner pipe and defines therewith a space in which said tubular walls are located, and a plurality of supporting bars extending radially between and being fixed to said inner pipe and outer cylinder, said bars forming a supporting frame means and said plurality of tubular walls being fixed at one of the ends thereof to said supporting frame means. 